An important aspect in implementing an ideal current source is achieving infinite output impedance. However, channel-length modulation of active devices associated with a current source generally limits the output impedance of the current source by causing the output current to be a function of the output voltage. Channel-length modulation is the effect of a pinchoff region forming before the drain of a transistor under a large drain bias. The pinchoff region shortens the channel region, and leaves a gap of uninverted silicon between the end of the formed inversion layer.
There are several conventional techniques for improving the limited output impedance of a current source. For example, stacked cascode devices have been implemented with a current source to limit voltage variations on the first stacked device of the current source. Also, operational amplifiers have been implemented within a feedback loop of a current source to force a fixed voltage across the first stacked device of the current source. However, each of these conventional techniques has drawbacks. The current source including stacked cascode devices requires a higher voltage headroom in order to provide enough saturation voltage for the stacked devices. The operational amplifier approach is only effective within the feedback loop bandwidth and, therefore, such an approach is not useful for high-speed low-power applications.
Channel-length modulation becomes a more serious problem in applications in which there are very large output swings with limited headroom and high linearity requirements. An example of such an application is an output driver (e.g., a current digital-to-analog converter) in a 10GBASE-T application in which the links are bi-directional and an outgoing signal is superimposed with an incoming signal, both having an amplitude of ˜2V peak-to-peak (totaling 4V peak-to-peak). In a case where the output driver is a current DAC, the current DAC is typically a 10 bit DAC having a 60 dB linearity requirement. While one can simply increase the supply voltage to increase the headroom, the increase in headroom comes at a high cost of power, as well as the need to implement inferior high voltage devices.